1. Field of the Invention
The present invention is generally related to magnetic sensors and, more particularly, to a magnetic sensor that can be accurately calibrated by selectively moving a magnet relative to a magnetically sensitive component and rigidly attaching the magnet in position when a desired relationship between the magnet and magnetically sensitive component is achieved.
2. Description of the Prior Art
Many different types of magnetic sensors are known to those skilled in the art. One particular type of sensor incorporates a biasing magnet that is associated with a magnetically sensitive component, such as a magnetoresistive element or a Hall effect element. Sensors which use biasing magnets respond to a change in the magnetic field provided by a permanent magnet when a ferromagnetic object moves into a detection zone. When sensors of this type are intended to be mass produced, the relative position between the magnet and the magnetically sensitive component should be accurately controlled so that ferromagnetic objects can be detected in an identical manner, regardless of the particular sensor used.
Sensors of this type can use either magnetoresistive elements or Hall effect elements. Magnetoresistive sensors are described in an article titled "Magnetoresistive Sensors" by B. Pant in the Fall 1987 issue of the Scientific Honeyweller. The article describes the use of magnetoresistive material in various sensor applications. It also discusses the resistance of the sensors and the change in resistance in response to an external magnetic field. Various design tradeoffs are dictated by the forces that compete to determine the direction of magnetization in a thin magnetoresistive film and these tradeoffs are discussed.
U.S. Pat. No. 5,041,784, which issued to Griebeler on Aug. 20, 1991, discloses a magnetic sensor with a rectangular field distorting flux bar. The sensor is used in measuring the position, velocity or direction of movement of an object having alternating zones of magnetic conductivity with a permanent magnet member having a pole face that faces the moving object and having an axis that is transverse to the direction of movement of the object. A ferromagnetic strip of high permeability is mounted on the face of the magnet coaxial with the magnet and having a length dimension in the direction of movement of the object which is greater than the width dimension transverse to the direction of movement.
U.S. Pat. No. 4,725,776, which issued to Onodera et al on Feb. 16, 1988, describes a magnetic position detector that uses a thin film magnetoresistor element that is inclined relative to a moving object. The detector employs magnetoresistive elements and detects magnetic teeth of an object to be detected. More specifically, a constant DC magnetic field is supplied to the magnetoresistive elements in such a way so as to avoid a nonlinear region of the DC magnetic field to permit the use of such elements in the regions exhibiting good linearity. The invention provides the DC magnetic field to the magnetoresistive elements by employing a simple structure wherein the magnetoresistive elements are arranged inclined relative to the magnetic field defined between a permanent magnet and the magnetic teeth.
U.S. Pat. No. 5,289,122, which issued to Shigeno on Feb. 22, 1994, discloses a magnetic sensor for detecting course and fine magnetic patterns. A plurality of sensing parts are deposited and formed on an element substrate in film form. The inside two of the sensing parts are connected in series to be used for a reading of a narrow pitch magnetic pattern and the outside two of the sensing parts are also connected in series to be used for reading a wide pitch magnetic pattern. The connection of the sensing part is carried out by using terminals and wires. The terminals are provided so as to short circuit the ends of the sensing parts.
A review article titled "The Permalloy Magnetoresistive Sensors-Properties and Applications" by W. Kwiatkowski and S. Tumanski, in the 1986 issue of The Institute of Physics, presents a review of the properties and applications of the permalloy magnetoresistive sensors of magnetic fields. It gives information on the manufacturing and biasing methods used in conjunction with the sensors. The basic parameters, which include sensitivity, dimensions, linearity, resolution and transducer errors are analyzed and various methods of improving these parameters are discussed. The examples of permalloy microsensors, miniature sensors and large area sensors are also presented in the paper. The application of permalloy magnetoresistors to measuring magnetic fields and constructing electrical and nonelectrical transducers is described.
U.S. Pat. No. 5,128,613, which issued to Takahashi on Jul. 7, 1992, describes a method for inspecting magnetic carbonization in a nonpermeable material. A probe is described which comprises a magnet and a Hall element which are provided in a magnetically impermeable case. The Hall element is disposed at the midportion between the two poles of the magnet in parallel with the lines of magnetic flux. The presence of a carbonized portion in the member to be inspected and the depth of the carbonization are detected by passing DC current across the Hall element and detecting the Hall effect electromagnetic force produced between the two ends of the element which are opposed in a direction perpendicular to the flow of the current.
U.S. Pat. No. 4,853,632, which issued to Nagano et al on Aug. 1, 1989, describes an apparatus for magnetically detecting a position of a moveable magnetic body. The apparatus includes a three terminal magnetic field intensity sensing structure formed by a pair of magnetoresistors. The magnetic field intensity sensing structure is disposed opposite to a magnetic body that is arranged for movement relative thereto in a magnetic field and generates a first electrical signal of sinusoidal waveform in response to a change in the intensity of the magnetic field due to the relative movement of the magnetic body. This first electrical signal appears from the apparatus as a second electrical signal of rectangular waveform or of sinusoidal waveform having an amplified amplitude. Components of the circuit for shaping the waveform or amplifying the amplitude of the first electrical signal are integrally mounted together with the magnetoresistors on a substrate. Preferably, the shaping or amplifying circuit is in the form of a hybrid integrated circuit formed on the substrate.
U.S. Pat. No. 4,535,289, which issued to Abe et al on Aug. 13, 1985, discloses a device for measuring a position of a moving object. A detected member made of magnetic material is secured to the moving body and E-shaped magnetic is arranged adjacent the measured member transversed to the direction of motion. A Hall IC for converting the variation of magnetic flux density of the magnet to the variation of voltage is secured to an end of a central leg portion of the magnet. The measured member is an elongated bar and is provided with a series of projections on the elongated bar at both sides. The projections of both sides are arranged in staggered relation. The Hall IC is adjacent to an inner portion of one of the projections when the moving body travels in the elongated directions. The Hall IC produces an output having a waveform with a zero level interval between inverted waves.
U.S. Pat. No. 5,304,926, which issued to Wu on Apr. 19, 1994, describes a geartooth position sensor with two Hall effect elements. The position sensor has two magnetically sensitive devices associated with a magnet. The sensor is disposable proximate a rotatable member having at least one discontinuity in its surface. The two magnetically sensitive devices, such as Hall effect transducers, each provide output signals that represent the direction and magnitude of the magnetic field in which its respective transducer is disposed. An algebraic sum of the first and second output signals from the magnetically sensitive devices is produced as an indication of the location of the rotatable member that is disposed proximate the sensor.
High resolution magnetic gear tooth sensors typically require calibration for specific applications. One such type of application is a complementary target scheme such as that disclosed in U.S. patent application Ser. No. 08/032,883 which was filed by Wu on Mar. 18, 1993 and assigned to the Assignee of the present application.
This type of gear tooth sensor can use a magnetic system which requires the adjustment of the magnet in order to achieve magnetic null for proper system operation. Zero crossing detection is used in the conditioning circuitry to achieve the highest possible accuracy. Previously, prototype devices of this type have been calibrated around the zero crossing by simply adjusting the biasing magnet behind a magnetoresistive sensor until the bridge output is equal to its original bridge null prior to the introduction of the bias. This analog output of the bridge had been constantly monitored during a first portion of a calibration procedure which was completed once the original null voltage had been reached. Adjustment was made without the presence of a complementary target placed within the detection zone of the magnetic sensor. The last portion of the calibration procedure was achieved by adjusting the circuit null to coincide with the originally measured bridge null. Although this type of calibration scheme has been generally successful for providing prototype sensors when the sensors had separate bridge and circuitry components, it also provides several serious disadvantageous limitations when flexibility, manufactureability and produceability are considered. For example, a two-step calibration procedure was required and this is not cost effective. A scheme of this type not only suggests longer calibration cycle time, but also increases the required equipment costs necessary to accommodate dual adjustments. The second adjustment under this type of calibration procedure can be omitted if the bridge and the circuitry are matched by appropriate trimming procedures during the IC probing process. In addition, since most integrated IC sensors are provided with supply, negative and output terminations, two additional interconnections are necessary to provide pin outs for the sensor IC in order to measure the actual bridge output. The bridge output, or differential voltage across the magnetoresistive bridge, must be made available in order to monitor the bridge null voltage. This results in increased space required for the IC and additional costs, assuming that the bridge output would require buffering that is separate from the control circuitry and additional bonding pads for the necessary wiring connections. In addition, a disadvantageous increase in EMI and RFI susceptibility can result because the bridge output connections are exposed to the outside world through wire bonds, pins, traces and pads which act like antennae that receive the various types of interference signals.
Continual monitoring of the analog output from the bridge can also result in longer calibration cycle times. This type of calibration procedure requires that the bridge output be read, compared to a previously determined null voltage that varies from sensor to sensor, and tripped through a digital logic circuit that ultimately ceases the movement of a magnet relative to the magnetically sensitive component. In addition, the analog bridge output is significantly lower than system control voltages since it is measured in millivolts. Noise susceptibility in a calibration system of this type would naturally be increased unless the analog output is sufficiently filtered. Such filtering also increases the calibration cycle time.
In the manufacture of magnetic sensors, it is important to calibrate the sensor so that it provides a predictable signal when placed in a particular position relative to a ferromagnetic object, such as a gear tooth. In automotive applications, it is particularly important to calibrate the sensor so that it reacts predictably with a preselected signal of known magnitude when an edge of a gear tooth passes through a certain position within the detecting zone of the sensor. If the sensor isn't properly calibrated, it can provide its output signal in either a premature or delayed manner and therefore not be useable in conjunction with automotive engines which require precise timing signals. When magnetic sensors are manufactured, the calibration procedure typically requires two separate processes to be followed. One process is the calibration of the magnetically sensitive component in relation to associated circuit components. In addition, the position of a permanent magnet in relation to the magnetically sensitive component must be held precisely. These calibration procedures can be both costly and time consuming. In addition, the required circuit configuration that permits the appropriate calibration measurements to be performed sometimes leads to other problems regarding susceptibility to electromagnetic interference, or EMI, and radio frequency interference, or RFI. It would therefore be beneficial if a magnetic sensor could be provided in which calibration procedures are simplified by permitting the magnet to be moved relative to the magnetically sensitive component during the calibration process and then rigidly holding the magnet in position until it can be permanently fixed within a sensor structure.